Gelatin is obtained by the partial hydrolysis of collagen, the chief protein component in skins, bones, hides, and white connective tissue of the animal body. Gelatin is produced by either acid conditioning or by alkaline or lime conditioning followed by hot water extraction of the gelatin from the collagen material
Most acid treated gelatin is made from pork skins yielding grease as a byproduct which is also marketable. The process employed includes comminuting the skins (collagen material), washing the skins to remove extraneous material, treating with a 1-5% acid solution of mineral acid in water, neutralization of the skins by adjustment of the pH with a base followed by water washing and finally water extraction in 7-8 batches at increasing temperatures up to about 212.degree. F. Each extraction is typically conducted for approximately 2-4 hours. Grease is removed from the gelatin extract which is then filtered, concentrated, chilled and dried by air on wire mesh belts. The dry gelatin is then ground and blended to specification.
Lime treated bones but also hides and skins are comminuted and placed in liming tanks for 3-16 weeks. Material is then washed from 15-30 hours to remove the with available acids. The gelatin is then extracted as in the acid conditioned process. Kirk-Othmer "Encyclopedia of Chemical Technology", Third Edition, vol. 11, pp. 711-715, John Wiley and Sons, N.Y. 1980.
In an early U.S. Pat. No. 2,557,871 a multiple cook process was compared to a single step extraction of a dried, comminuted collagen product. The comparison employed a first cook-out at a temperature of between 120.degree. F.-150.degree. F. for about 3-5 hours. Further "successive" cook-outs where accomplished at progressively increasing cooking times and temperatures until the last extraction was at boiling. The gelatin solution formed was removed, filtered, evaporated, dried and ground.
In example 1 of U.S. Pat. No. 2,557,871 the first cook-out was made at 140.degree. F. for four hours followed by successive cook-outs at 155.degree. F., 170.degree. F., 185.degree. F. and finally 210.degree. F. for periods of four hours each for the first four cook-outs and ten hours at the cookout for boiling temperature. The gel strength of each cook-out diminished substantially beginning at 285 bloom and being reduced to 50 bloom for the cook-out at boiling temperature. These conventional cook-outs were then compared to the one cook-out process employing the dried, comminuted collagen product . The one cook-out process was found to be superior to any one or all of the conventional cook-outs which varied with time and temperature.
In U.S. Pat. No. 2,024,683 entitled "Gelatin Product and Method of Making Same", organic acids are found to be preferable in treating pig skin to reduce the detrimental effect of low pH caused by mineral acids on the quality of the extracted gelatin.